Automatic volume control system



O. C. MONTGOMERY AUTOMATIC VOLUME CONTROL SYSTEM March 30, 1954 FiledSept. 23, 1949 N mPlrL JNVENToR. o. C .MONTGOMERY BY t INE-IEE( L. MU mmOa mmPJ-L Patented Mar. 30, 1954 AUTOMATIC VOLUME CONTROL SYSTEM Orin C.Montgomery, Bartlesville, Okla., assignor to Phillips Petroleum Company,a corporation of Delaware Application September 23, 1949, Serial No.117,478

14 Claims. 1

This invention relates to an improved automatic volume control systemfor seismic ampliiiers. In another aspect, it relates to a lter circuitfor use with such ampliers.

In seismic surveying, an explosive charge is nred at a predeterminedlocation in the earth, known as the shot point, and waves are propagatedthrough the earth by this explosion. A portion of these waves traveldownwardly through the earth, are reflected from subterranean strata,and the reiiected waves are picked up by mechano-electrical transducers,such as geophones, which produce an electrical voltage representative ofthe reiiected waves. By measuring the time interval between the :firingof the charge, referred to as the shot instant, and the time at lwhichthe reflected waves are picked up by the geophones, valuable informationcan be obtained as tothe nature and location of the subterranean strata.In ordinary practice, the electrical output of the transducer orgeophone is amplified and the amplified voltages are used to actuate arecorder unit.

Many diiiculties have been encountered in maintaining the properamplifier gain during the recording period. The rst signal produced bythe geophone as a result of the explosion is oftentimes quite weak, butthe signal level then quickly builds up to an amplitude of as much as10,000 times the original amplitude, and then rapidly decreases again.

In order to make recordings from a number of different points on anarrow width or photon graphic paper, it necessary that the amplier gainbe regulated so that the recorder input does not vary beyond preselectedlimits, despite the aforementioned very large changes in signalstrength. Gtherwise, the recorder trace will move oi the record when thesignal amplitude is high.

@ne method heretofore used is an expander action volume control, inwthich the amplifier is started out at a very low level and is permittedto increase at a predetermined rate, starting at the shot instant. Thisdoes not provide the proper amplifier gain-signal strength relationunless several trial recordings are made to properly adjust theequipment. In addition, the expander action volume control does notprovide for maximum gain of the weak first signals which, accordingly,are not properly recorded.

Another method previously used involves an automatic volume controlwhich adjusts the gain of the amplifier so that the outlet amplituderemains about the same with large changes in insignals. This. per-mitsthe amplifier gain to be at a maximum at the beginningof the record andto be adjusted automatically with respect vfrom the input voltages.

to the input signal throughout the record. It is necessary to cause thegain to change at a slow enough rate so the wave shape of the lowestfrequencies recorded will not be changed to a very great extent. As aresult, a denite time interval is required for the amplifier gain to bereduced after the rst high amplitude signals. Thus, this system permitsweak initial signals to appear at a useable amplitude, which is not trueof the expander method, but it cannot reduce the gain rapidly enough forproper recording of early reflections following the rst high amplitudesignals.

It is an object of this invention to provide an automatic volume controlsystem which provides the advantages of both previous types of systemswithout their` disadvantages.

It is a further object of the invention to provide a combination of longand short time constant systems which are controlled by biased electrontubes with the result that the time constants of the lter unitsfollowing the triodes are changed as the automatic volume control systemis operated.

It is another object to provide a system for controlling the gain of theamplifier tubes by applying control voltages to the suppressor grids andto utilize a lter circuit connected to the suppressor grid so as toimprove operation of the amplifier circuits.

It is another object of the invention to provide an amplifier which isprevented from operating during the period prior to ring of the shot.

lt is a still further object to provide such an amplier which is easy toconstruct, utilizes standard circuit components, and is economical tobuild and manufacture.

Various other objects, advantages and features of the invention willbecome apparent from the following detailed description taken inconpunction with the accompanying drawings, in which:

Figure 1 is a block diagram showing the automatic volume control systemof this invention; and

Figure 2 is a schematic circuit diagram illustrating the operation ofthis invention.

Referring now to the drawings in detail, and

particularly to Figure 1, the seismic system includes an amplier l0which may have one or more stages, the output of which is fed through alter il to a power amplifier l2, which, in turn, feeds a suitablerecording device, not shown. In this manner, the seismic signals areincreased in amplitude to such an extent as to readily operate arecording device, While the filter removes certain undesirablecomponents A portion of the output of power amplifier I2 is fed to ashort time constant automatic volume control circuit I3 and to a longtime constant automatic volume control circuit I4. The automatic volumecontrol circuith I3 and I4 operate in conjunction with filter i5 toregulate the gain of amplifier I0 so as to insure that the poweramplifier output voltage maintains a sufficiently constant amplitude asto be properly recorded.

The long time constant circuit gradually increases the gain of theamplier during the reoording period to compensate for the relativelygradual decrease in signal intensity resulting from the progressivedecrease in amplitude of the seismic waves, as the recording periodprogresses. The short time constant circuit operates very quicklywhen-there is an increase in amplitude of the signal above apredetermined level, as at the very beginning of the recording period.Since the short time constant functions only in response to signals ofhigh enough amplitude as to drive the recorder trace off the chart, itdoes not interfere with operation of the circuit after the initial highamplitude wave and, hence, has no tendency to distort the wave form ofthe latter part of the recording.

Referring now to Figure 2, the novel automatic volume control circuitsare operatively connected to pentodes 28 and 2i which are, respectively,the rst and second stage amplier tubes of the unit I0, Figure 1. lThesetubes are connected in a conventional amplifier circuit which will bedescribed in detail hereinafter, and the output of tube 2l is fedthrough the filter Il and, thence, to the power amplier I2 whichincludes a pentode 22, Figure 2, as its output tube. The output circuitof pentode 22 includes a coupling condenser 23 for connecting the anodeto a primary winding of an output transformer 24, and the secondarywinding of this transformer is coupled to the input circuit oi asuitable recorder, not shown, which produces traces on a recordingmedium representative of the amplitude of the amplied seismic signalspassing from the output circuit of power amplier I2. An operatingpotential is supplied to the anode of tube 22 from a positive terminal25 of a suitable power source, this terminal being connected throughseries resistors 28, 21 to the anode, and a grounded lter condenser 28is connected to the junction between the resistors 23 and 21.

In accordance with the invention, a portion of the output of poweramplifier 22 is fed through a lead 33 and coupling condensers 3i, 32 tothe respective control grids of tubes 33 and 34. Zin the example shown,the control tubes are triodes and, accordingly, each of them has ananode, a cathode, and a control grid. It will be apparent that tubeswith more electrodes, such as tetrodes, or pentodes, may be substitutedfor the triodes without departing from the spirit and scope of theinvention.

The tube 33 is biased substantially to cut off and the tube 34 is biasedseveral volts beyond cut o so that tube 33 becomes conductive when arelatively weak signal passes through power ampliiier I2 while tube 34does not become conductive unless a signal of relatively high amplitudeis passed through the power amplier i2. To this end, a bias network isprovided for tbe tubes 33 and 34, this network including a lead 35interconnecting the cathodes of the tubes, and -aunit including apotentiometer A35, a resistor 31, and a potentiometer 138, all connectedin series. A bias battery 39 has its positive terminal connected to lead35 and its negative terminal connected to the junction between resistor31 and potentiometer 38 while a battery 40 has its negative terminalconnected to lead 35 and its positive terminal grounded at 4I. A gridresistor 42 is connected between the control grid of triode 33 and thearm of potentiometer 36 while a grid resistor 43 is connected betweenthe control grid of triode 34 and the arm of potentiometer 33. It willbe apparent that the bias may be adjusted in the manner stated byvarying the setting of potentiometers 36 and 38 in such fashion thattube 33 is biased to cut oi and tube 34 is biased a few volts beyond cuton. The bias voltage may be tested at terminals 44.

The anode of tube 33 is connected through a resistor 45 and a lead -46to the suppressor grid of tube 2| and condenser 41 is connected betweenthe anode of tube 33 and ground. The anode of tube 34 is connecteddirectly through lead 46 to the suppressor grid of tube 2l, and lead 46is also connected to grounded condenser 43 and a grounded resistor 49.The voltage drop across resistor 49 may be tested at terminals 50. Thesuppressor grids of tubes 20 and 2l are interconnected by a resistor 5Iand a grounded condenser 52 is connected to the suppressor grid of tube20.

It will be apparent that condenser 43 and resistor 49 constitute afilter which operates upon the output voltage produced by tube 34.Preferably, condenser 48 has a capacitance of about 2 microfarads, andresistor 49 has a value of 200,000 ohms.

Similarly, the condenser 41 and the resistor 45, in conjunction with theparts already mentioned, constitute a second filter responsive to theoutput of tube 33. Condenser 41 preferably has a capacitance of onemicrofarad, and resistor 45 has a value of 50,000 ohms. The time takento charge lter 48, 49 is determined by the internal resistance of tube34 and the capacitance of condenser 48. Due to the circuit constantspreviously given, this lter is described as having a relatively shorttime constant and control voltages passed by tube 34 act upon theautomatic volume control circuit in a relatively brief interval of time.However, signals from tube 33 require a substantially longer time toaffect the automatic volume control circuit, as signals from this tubemust pass through resistor 45 and charge condenser 48 in addition tocharging condenser 41 before they affect the automatic volume controlcircuit. Accordingly, the lter defined by tube 33 and parts 45, 41, 48and 49 constitutes a filter of relatively long time constant and signalspassing through this path require a relatively long time interval beforethey affect the automatic volume control circuit.

The filter formed by resistance 5I, which preferably has a value of 1.3megohms, and condenser 52, which preferably has a capacitance of .1microfarad, has been found to provide improved operation of the circuitthrough control of the interrelation between the bias potentialssupplied to the suppressor grids of tubes 20 and 2|.

In accordance with the invention, I also provide a bias battery 55, thenegative terminal of which is connected through normally closed contacts56 of a relay 51 and a resistor 53 to lead 43 and the suppressor gridsof tubes 20 and 2i. The winding of relay 51 is connected in the anodecircuit of a gas lled triode 60, the anode being connected through therelay winding to a positive power, supply terminal 6l, and the cathodebeing grounded at 62. The control grid of triode. 50 is connectedthrough the secondary winding of ak transformer 63 to the negativeterminal of a bias battery 64, the positive termi.- nal of which isgrounded at 6.2. The primary winding of transformer E3 is adapted forconnection to a time break circuit of the type shown in the copendingapplication of de Boisblanc and Bondurant, Serial No. 628,310, now U. S.Patent 2,470,846, entitled Seismograph Blaster Time Breaker Circuit. Theoperation of this circuit is such as to impress a voltage pulse upontransformer E3 at the exact instant when a blasting charge is detonatedto produce seismic waves. This voltage pulse is thereupon eiective to retube 60 and open contacts 56, thereby removing bias battery 55 from thecircuit. The plate circuit of the triode 60 is interrupted when the mainpower supply is turned off after the recordin period to restore tube 6Flto non-conductive condition.

The operation of the novel automatic volume control system will now beapparent to those skilled in the art. Before the charge is detonated,the. contacts of relay 5T are closed and bias battery 55 impresses apotential upon the suppressor grids of tubes 20 and 2i which issufiiciently negative as to prevent amplication of noise signals bythese tubes. Although it is preferred that this potential be applied tothe suppressor grids, it is to be understood that the bias may beapplied to other electrodes, such as the control grid, if desired.

When the charge is detonated, a pulse is ap plied to transformer 63 withthe result that gas tube 60 becomes conductive and relay 5l isenergized, thereby opening contacts 56 and re moving the bias from thecontrol electrodes of tubes 20 and 2|. When gas tube 66 is fired in thismanner, it remains conductive until the end of the recording period,that is, until interruption of the main power supply circuit by which apositive operating potential is applied to the anode of tube 60.

After removal of the bias, the seismic signals picked up by theseisrnometer are amplified by tubes 20, 2l and their associatedcircuits, the amplier voltages passing successively through iilter Il,Figure l, tube 22 of power amplier i2,

action is applied by the voltage developed through tube 3/3 which feedsshort, time constant lter 48 and 49, this. sudden increase in biasvoltage quickly biasing the tubes 20 and 2i to reduce the gain to suchan extent as to provide the desired useable output from power amplifier22.

After the initial signal of high amplitude is recorded. the voltagessubsequently impressed upon tubes 33 and 34 are not of suiicientmagnitude as to overcome the bias on tube Ell. Accordingly, this tubebecomes inoperative during the remainder of the recording period and thegain level of the amplier is thereafter controlled entirely by the tube33. As the signal amplitude decreases during the recording period, thebias voltage produced by tube 33 likewise decreases therebyprogressively increasing the gain or arnplifier tubes Zd and 2l tomaintain a substantially constant amplitude during the remainder of therecording period. Due to the relatively long time constant of the lterassociated with tube the shape or wave form of the seismic signals isnot appreciably altered during the recording period so that the recordertrace accurately fol" lows the shape of the incoming seismic signals.

It will be apparent that my improved system reduces the sensitivity ofthe amplier before tiring of the seismic charge, thereby preventing thegalvanometer from recording noise on the trace. Thereafter, the twovolume control circuits cooperate to provide a useable and substantiallyconstant amplitude signal at the recorder despite large variations inamplitude of the coming seismic signals. Distortion is effectively fromproducing too high an amplitude on the the output transformer 24 to therecorder equipment. The signals which are amplified in this mannerconsist of initial rst breaks, or signals of very low amplitude. Thesignal amplitude then may build up to 10,000 or more times its originalvalue, after which it decreases gradually until the end of the recordingperiod. When the initial weak signals are received, no bias is appliedto the tubes 20 and 2l and the automatic volume control circuits areinoperative. results from the fact that tube 36 is biased near cut offwhile tube 23 is biased a few volts beyond cut off so that no automaticvolume control voltage is passed by these tubes. When signals of veryhigh amplitude are picked up shortly j Til recorder trace. The lter 5l,52 correlates the control action of tubes 20 and 2i and thereby providesa further improvement in operation of the system.

Figure 2 also shows the detailed circuits of the amplifier it, filteril, and power ampi 1 Figure 1. These circuits include trans 65, theprimary winding of which is conne input terminals t6 and 6l, and the secwinding of which is connected to ground al and to the control grid ofpentode secondary winding is shunted by condens and a plurality ofseries connected resistances le, these resistances being connected incircuit with a switch 'ii so as to vary the amplitude the signals fed tothe ampliiier. When the signal to noise ratio becomes too low, switch'il actuated to reduce the amplifier gain with the result that strengthof the seismic signals must be increased by increasing thev size of theein plosive charge to maintain a satisfactory signal to noise ratio.

Except for the connection of its suppressor' grid to the describedautomatic volume control circuit, each pentodett and 2i is connected ina conventional amplifier circuit. To this el the cathode of tube 20 isconnected to gr through a bias resistor 'i2 which is shunted a filtercondenser lt. The screen grid or" tube 2li is connected to a positivepower supply ter minal lei by a voltage dropping resistor i3 and toground by a filter condenser l5. The anode of tube 2i! is connected tothe control grid ci tube 2| by a coupling condenser 'lo and to apositive power supply terminal 'il by voltage dropping resistors 18. Agrid resistor 19 is connected between the control grid of pentode 2l andground while the cathode of this tube is connected to ground through abias resistor 30 which is shunted by a filter condenser 8 I. The anodeof tube 2| is connected through a voltage dropping resistor 82 to thejunction between resistors 18, this junction being grounded by a by-passcondenser 83, and through a coupling condenser S4 to the filter Il.functions to amplify the seismic signals fed to input terminals 66 andB1, the amplifier gain being controlled by the automatic volume controlcircuit in the manner previously described in detail herein.

The filter ll consists of a high pass section 85 and a low pass section86. High pass section 85 includes a set of condensers 81, a set ofcondensers 83, a switch B9, and a dual inductance, the sections :it and9i of which are wound upon a common magnetic core. The switch 89selectively connects one of the condensers 81 in series with inductancesection Sl and one of the condensers SS or, alternatively, switch 89 maymerely connect the inductance sections 9u and 9i in the circuit. In anycase, inductance section et is connected between one terminal ofinductance section Si and ground. It will be apparent that the pass bandcharacteristics of the filter may be adjusted at will by the switch S9to alter the cutoff frequency of the filter. The two section inductanceSG, 9i provides excellent cut-off characteristics for the filter andgreatly improves its efficiency, as compared with high pass filterspreviously utilized. The output of high pass lter 85 appears at aterminal 93 which is connected to ground through a resistance Sii and tolow pass filter section 86 through a coupling resistance 95.

The low pass lter 85 includes two series 9S and @l of condensers whichare connected, respectively, to multi-position switches 98 and 99, thearms of these switches being connected, in turn, to opposite terminalsof an inductance |09. Accordingly, the low pass filter section includesseries inductance |00, each terminal of which is connected to groundthrough one of the condenser-s et or el'. It will be apparent that thecut-off frequency of this filter may be altered by suitable adjustmentof the switches 98 and 99 to change the cut-oil frequency of the filter.

The output of low pass filter 86 is connected by a lead lili to thecontrol grid of pentode 22, and lead iti is also connected to groundthrough a grid resistor m2. The cathode of pentode 22 is connected toground through a bias resistor itt which is shunted by a by-passcondenser l Q4. The suppressor grid of tube 22 is connected directly toits cathode, and the screen grid of tube 22 is connected through avoltage dropping resistor 35 to positive power supply terminal 25 andthrough a filter condenser IGS to ground. The output circuit of pentode22 has already been described in connection with the automatic volurnecontrol circuit.

It will be apparent that the amplifier l0, and ilter Ii function toamplify the seismic signals fed to the input terminals t and El, theamplifier gain being regulated jointly by switch 'H and the automaticVolume control-circuits. lIn addition, the lter I l limits the outputsignals to freduencies within a predetermined range, signals of higherthan a predetermined frequency being eliminated by filter sections 86and signals of The circuit thus far described 8 l lower than apredetermined frequency being eliminated by filter section 85. The novelcircuit of lter section permits a very sharp cutoif to be obtained ascompared with high pass filters heretofore utilized.

While the invention has been described in connection with a present,preferred embodiment thereof, it is to be understood that thisdescription is illustrative only and is not intended to limit theinvention, the scope of which is defined by the appended claims.

Having described my invention, I claim:

1. An automatic volume control unit comprising, in combination, anamplifier tube having a control electrode for regulating its gain, apower amplifier stage fed by said amplifier tube, a pair of controltubes each having an anode, a cathode, and a control grid, means forsupplying operating and bias potentials to said control tubes so thatone tube is biased substantially to cut off and the other tube is biasedbeyond cut off, means for feeding a portion of the power amplifieroutput to the control grid of each control tube, a filter having arelatively long time constant fed by said one control tube, a filterhavingr a relatively short time constant fed by said other control tube,and means for feeding back the output of both filters to said controlelectrode.

2. An automatic volume control unit comprising, in combination, a twostage ampliier, each stage including an amplifier tube having a controlelectrode for regulating its gain, a power amplifier stage fed by thesecond amplifier tube, a pair of control tubes each having an anode, acathode, and a control grid, means for supplying operating and biaspotentials to said control tubes so that one tube is biasedsubstantially to cut off and the other tube is biased beyond cut off,means for feeding a portion of the power amplifier output to the controlgrid of each control tube, filter means connecting the anodes of bothcontrol tubes with the control electrode of the second amplifier tube,said filter means having a relatively long time constant with respect tosignals fed from the anode of said one control tube to saidlast-mentioned control electrode, and said filter means having arelatively short time constant with respect to signals fed from. theanode of said other control tube to said last-mentioned controlelectrode, and a filter interconnecting said control electrodes, saidlast mentioned lter including a resistance connected between the twocontrol electrodes, and a condenser connected between the controlelectrode of the rst amplier tube and ground.

3. An automatic volume control unit comprising, in combination, a twostage amplicr unit,

each stage including an amplier tube having a control electrode forregulating its gain, a power amplifier fed by the last stage of saidamplifier, a pair of control tubes each having an anode, a cathode, anda control grid, means for supplying operating and bias potentials tosaid control tubes so that one control tube is biased substantially tocut off and the other tube is biased beyond cut oi, means for feeding aportion of the power amplifier output to the control grid ci f centroltube, a filter having a relatively long time constant including aresistor and a condenser each having one terminal connected to theano-:ie of said' one control tube,Y the other terminal of the condenserbeing grounded and the other terminal of the resistor being connected tothe control electrode of the second amplier stage, a

lter having a relatively short time constant infcluding a condenserconnected between the anode of said other control tube and ground and aresistance shunting said condenser, a lead connecting the anode of saidother control tube to the control electrode of the second amplifierstage, and a filter interconnecting said control electrodes, said lterincluding a resistance connected between said control electrodes and acondenser having one terminal grounded and having its other terminallconnected to the control electrode of the rst amplier tube.

4. An automatic volume control unit comprising, in combination, a twostage amplifier unit, each stage including an amplifier tube having acontrol electrode for regulating its gain, a power amplifier fed by thelast stage of said amplifier, a pair of control tubes each having ananode, a cathode, and a control `grid, means for supplying operating andbias potentials to said control tubes so that one tube is biasedsubstantially to cut oi and the other tube is biased beyond cut on,means for feeding a portion of the power amplifier output to the controlgrid or each control tube, a nlter having a relatively long timeconstant including a resistor and a condenser each having one terminalconnected to the anode of said one control tube, the other terminal ofthe condenser being grounded and the other terminal of the resistorbeing connected to the control electrode of the second amplifier stage,a filter having a relatively short time constant including a condenserconnected between the anode of said other control tube and ground, and aresistance shunting said condenser, a lead connecting the anode of saidother control tube to the control electrode or the second amplifierstage, land a filter interconnecting said control electrodes, saidfilter inciu/ding a resistance 'connected between said controlelectrodes and a condenser having one terminal grounded and having itsother terminal connected to the control electrode of the firstamplifier' tube, a battery for impressing a cut off bias on both of saidcontrol electrodes', a relay having normally closed contacts in circuitwith .i

battery and said electrodes, a gas filled triode having the winding ofsaid relay connected in its anode circuit, means for applyingloperatingpotentials to the electrodes of said gas filled triode that itis normally biased beyond cut off, means for overcoming said bias onfiring of a seismic charge to cause said gas lled triode to becomeconductive, thereby to actuate said relay and i'emove the cut off biasfrom said control electrodes.

e. An automatic volume control unit comprising, in combination, anamplifier stage including a tube having a suppressor grid, the gain ofsaid amplieil being adjustable by control of the sup'- presser gridvoltage, a power lamplifier stage fed by said amplifier tube, a pair ofcontrol tubes each having an anode, a cathode, and a control means forsupplying operating and bias .entials to said control tubes Vso that onetube i sed substantially to cut orf and the other biased beyond cut off,means for feeding a portion of the amplier Signal to the control i ofeach control tube, filter means connecting e anodes of both controltubes with said suppresso-r grid, said lter means having a relativelylong time constant with respect to signals fed from the anode of saidone control tube to said last-mentioned control electrode, and said'filter having a relatively short time constant with respect to signalsfed from the anode of said other control tube to said last-mentionedcontrol electrode.

6. An automatic volume control unit comprising, in combination, a twostage amplifier unit, each stage including an amplifier tube having asuppressor grid, the gain of each stage being ad- :instable by Varyingthe voltage applied to said suppressor grid, a power amplifier fed bythe last stage of said amplifier, a pair of control tubes each having ananode, a cathode, and a control grid, means for supplying operating andbias potentials to said control tubes so that one tube is biasedsubstantially to cut off and the other tube is biased beyond cut off,means for feeding a portion of the power amplier output to the controlgrid of each control tube, a filter having a relatively long timeconstant including a resistor and a condenser each having one terminalconnected to the anode of said one control tube, the other terminal ofthe condenser being grounded and the other terminal of the resistorbeing connected to the suppressor grid of the second amplifier stage, alter having a relatively short time constant including a condenserconnected between the anode of said other control tube and ground, and aresistance shunting said condenser, a lead connecting the anode of saidother control tube to the suppressor grid of the second amplifier stage,and a filter interconnecting said suppressor grids, said nlter includinga resistance connected between said suppressor grids and a condenserhaving one terminal grounded and having its other terminal connected tothe suppressor grid of the first amplifier tube.

7 An automatic volume control unit for a seismic amplifier systemcomprising, in combination, a two stage amplifier unit, each stageincluding an amplifier tube having a suppressor grid, the gain of theamplifier being adjustable by varying the voltage applied to thesuppressor grid, a power amplifier fed by said two stage amplifier, abattery for impressing a cut off bias on both suppressor grids, a relayhaving normally closed contacts in circuit with said battery and saidsuppressor grids, a gas filled triode having the winding of said relayyconnected in its anode circuit, means for applying' operating potentialto the electrodes of said triode so that it is normally biased beyondcut o, means for overcoming said bias on ring of a seismic charge tocause said triode to become conductive, thereby to actuate said relayand remove the cut oi bias from said suppressor grids, and an automaticvolume control system for controlling the potential of said suppressorgrids after the relay is actuated, a pair of control tubes each havingan anode, a cathode, and a control grid, means for applying operatingand bias potentials to said control tubes so 'that one tube is biasedsubstantially to cut ofi and the other tube is biased beyond cut ofi",means for feeding a portion of the power amplifier output to the controlgrid of cach control tube, a filter having a relatively long timeconstant fed by said one control tube, a lter having a relatively shorttime constant fed by said other control tube, means for feeding back theoutput of both filters to the suppressor grid of the second amplifiertube, and a filter interconnecting the suppressor grids of saidamplifier tubes, said lastmentioned filter including a resistanceinterconnecting said suppressor grids, and a condenser having oneterminal grounded and having its other terminal connected to thesuppressor grid of the first amplier stage.

8.1An automatic volume control unit comprising, in combination, anamplifier tube having a control electrode for regulating its gain, afilter fed by said amplifier tube for rejecting signals of higher than afirst predetermined frequency, and for rejecting signals of lower than asecond predetermined frequency, a power amplifier stage fed by saidfilter, a pair of triodes each having an anode, a cathode, and a controlgrid, means for supplying operating and bias potentials to said triodesso that one triode is biased substantially to cut off and the othertriode is biased beyond cut ofi, means for feeding a portion of thepower amplifier output to the control grid of each triode, means forfiltering the output of both of said triodes and feeding the filteredoutput back to said control electrode.

9. A high pass filter comprising, in combination, a set of inputterminals, a set of output terminals, a circuit interconnecting oneinput terminal and one output terminal, said circuit including a firstcondenser, an inductance and a second condenser, all connected inseries, a second inductance having one terminal thereof connected tosaid first inductance, and means interconnecting the other inputterminal, the other output terminal, and the other terminal of saidsecond inductance, said inductances having a common magnetic core.

l0. A high pass filter comprising, in combination, a pair of inputterminals, a pair of output terminals, a circuit connecting one inputterminal to one output terminal, said circuit including a set ofcondensers, a first inductance, a switch for selectively connecting oneterminal of said first inductance in series with one of said set ofcondensers and said one input terminal, a second set of ccndensers, aswitch for selectively connecting the other terminal of said firstinductance in series with one of said second set of condensers and saidone output terminal, a second inductance having one terminal thereofconnected to said first inductance, said inductances having a commonmagnetic core, and means interconnecting said other terminal of thesecond inductance, the other input terminal, and the other outputterminal.

11. An automatic volume control unit cornprising, in combination, anamplifier tube having a control electrode for regulating its gain, afilter fed by said amplifier tube for rejecting signals of higher than afirst predetermined frequency, and for rejecting signals of lower than asecond predetermined frequency, said filter including a high passsection consisting of a first condenser, an inductance, and a secondcondenser, all connected in series, and a second inductance connectedbetween one terminal of said first inductance and ground, saidinductances having a common magnetic core, a power amplifier stage fedby said filter, a pair of triodes each having an anode, a cathode, and acontrol grid, means for supplying operating and bias potentials to saidtriodes so that one triode is biased substantially to cut off and theother triode is biased beyond cut oli, means for feeding a portion ofthe power amplifier output to the control grid. of each triode, meansfor filtering the output of both of said triodes and feeding the ilteredoutput back to said control electrode.

12. An automatic volume control unit comprising, in combination, anamplifier tube having a control electrode for regulating its gain, afilter fed by said ampliner tube for rejecting signals of higher than afirst predetermined frequency, and for rejecting signals of lower than asecond predetermined frequency, said filter including a high passsection including two sets of condensers, and a dual inductance, meansfor selectively connecting one condenser of each set in series with onesection of said inductance, and means for grounding the other section ofsaid inductance, a power amplifier stage fed by said filter, a pair oftriodes each having an anode, a cathode, and a control grid, means forsupplying operating and bias potentials to said triodes so that onetriode is biased substantially to cut off and the other triode is biasedbeyond cut off, means for feeding a portion of the power amplifieroutput to the control grid of each triode, means for filtering theoutput of both of said triodes and feeding the iiltered output back tosaid control electrode.

13. An automatic volume control unit comprising an amplier including atube having a control electrode for regulating its gain, a pair ofcontrol tubes each having at least an anode, a cathode, and a controlgrid, means for supplying operating potentials and adjustable biaspotentials to said control tubes, means for feeding a signal from saidamplier to the control grid of each control tube, filter meansconnecting the anodes of both control tubes with said control electrode,said filter means having a relatively long time constant with respect tosignals fed from the anode of said one control tube to saidlast-mentioned control electrode, and said filter means having arelatively short time constant with respect to signals fed from theanode of said other control tube to said last-mentioned controlelectrode.

14. An automatic volume control unit comprising, in combination, anamplifier having a plurality of stages, each of two of said stagesincluding an amplifier tube having a control electrode for regulatingits gain, a pair of control tubes each having at least an anode, acathode, and a control grid, means for supplying operating potentialsand adjustable bias potentials to said control tubes, means for feedinga signal from said amplifier to the control grid of each tube, filtermeans connecting the anodes of both control tubes with the controlelectrode of one of said amplifier tubes, said filter' means having arelatively long time constant with respect to signals fed from the anodeof said one control tube to said last-mentioned control electrode, andsaid filter means having a relatively short time constant with respectto signals fed from the anode of said other control tube to saidlastmentioned control electrode, and a ilter interconnecting saidcontrol electrodes, said lastmentioned filter including a resistanceconnected between the two control electrodes, and a condenser connectedbetween the control electrodes of the rst amplifier tube and ground.

ORIN C. MONTGOMERY.

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